Modification And Electrochemical Properties Of Electrode Materials On All Vanadium Redox Flow Battery

All vanadium redox flow battery (VRFB for short) has aroused wide concern as it possesses advanced characteristics such as high energy efficient, long cyclic life and environmental friendly. VRFB has received the extensive attention as one of large-scale high efficient energy storage devices.Electrode is one of the major critical components in VRFB; therefore, it is very important to study the electrode materials. Graphite felts (GFs) have been used in various fields such as catalysis of redox reactions, electrochemical capacitors, electrochemical sensors, etc., due to their unique physical and chemical properties. But Graphite felts have been proven to show a poor electrochemical activity. In this paper, based on the review of research and development of VRFB and its electrode materials, GFs were used as electrodes in VRFB and their electrochemical performance was investigated by different material characterization methods and electrochemical techniques, such as SEM, XRD, BET, XPS, cyclic voltammetry and electrochemical impedance spectroscopy etc.In the third chapter of this dissertation, amino-fluorination of graphite felts are used as the positive electrodes of all-vanadium redox flow battery (VRFB), which are surface modified by ammonium hydroxide and hydrofluoric acid to develop the cell electrochemical properties. The electrochemical performances and the catalytic activity of amino-fluorination of graphite felts were characterized by chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy. In addition, the stability of WA-F was tested in fuel cells with serpentine flow field. The experimental results showed that N-and F-containing surface functional groups are introduced on the surface of graphite felt and the specific surface area is also increasing. It was beneficial for positive electrode reactions of VRFB to treat with different ways, and the best electrochemical performance was obtained with WA-F electrode. A VRFB was constructed using WA-F electrodes as positive and negative electrodes. The preliminary results, obtained by charge/discharge test at a current density of20mA/cm2, illustrated that the VRFB has good stability and electrochemical performance. The current efficiency, voltage efficiency and energy efficiency were96%,78%and76%, respectively.In the fourth chapter, WO3was electrodeposited on the surface of GFs to investigate the electrochemical performance applied in VRFB. It was found that the WO3/GF is beneficial for the VO2+/VO2+and V2+/V3+redox reactions and improves hydrogen evolution reaction. The preliminary results, obtained by charge/discharge test at a current density of20mA/cm2, illustrated that the VRFB of WO3/GF used as negative electrode had better stability and electrochemical performance than the VRFB of WO3/GF used as positive electrode. The current efficiency, voltage efficiency and energy efficiency were96%,81%and76%, respectively.In the fifth chapter, the highly hydroxylated graphite felts (GFs) were obtained by treating in mixed acids (V(HNO3)/V(H3PO4)=3:1) for different time at80℃, and used as the electrodes for all vanadium redox flow battery (VRFB). The results showed that the introduction of-OH groups onto GF surface can significantly improve its electrocatalytic activities for VRFB reactions. This can be ascribed to the enhancement of interaction between vanadium ions and-OH functional groups formed on the GF surface, which facilitate the electron transfer of vanadium ions at the GF surface. The appropriate time for treating GF in the mixed acids is8h, and the GF structure will be destroyed if the time further prolongs. The VRB using AGF-8(GF treated in the mixed acid for8h) as electrodes exhibits excellent performance under a current density of20mA/cm2. The average current efficiency is about97%.